1. Field of the Invention
The present invention relates to inertial sensors. More particularly, this invention pertains to an improved design for a ring laser gyroscope.
2. Description of the Prior Art
The multioscillator has been proposed as a means for overcoming the "lock-in" problem in ring laser gyroscopes. As is well known, lock-in refers to the tendency of counterpropagating beams to lase at a single frequency, or lock point, at low input rotation rates. As such, ring laser gyroscopes are essentially insensitive to rotation rates below known characteristic thresholds. The range of input rates over which the gyro gives no output is known as the "dead band". One common means of overcoming this insensitivity is known as mechanical dither and involves the application of a bounded oscillatory motion to the gyro frame. In this manner the gyro is continually swept through the dead band and the effects of lock-in can be greatly reduced. The shortcomings of the mechanically dithered gyroscope are well recognized in the art.
In essence, the multioscillator operates as a pair of two-mode ring laser gyroscopes that share a single cavity. The multioscillator light cavity sustains a substantially left circularly polarized (LCP) beam pair, comprising one beam circulating in the clockwise direction and the other in the counterclockwise direction and a substantially right circularly polarized (RCP) beam pair also comprised of counterprogagating beams. Ideally, each beam pair acts independently as a two-mode ring laser gyroscope and senses body rotation by means of the Sagnac effect.
In order to achieve independent operation of these two gyroscopes within the same cavity, a means is applied to the cavity to ensure that the two beam pairs, one pair of LCP light and the other of RCP light, operate about different frequencies. This separation of frequencies is known as "reciprocal splitting" and is typically on the order of a few hundred MHz. Early multioscillator designs achieved the necessary reciprocal splitting by the placement of a suitably aligned optically active element in a three- or four-mirrored cavity as is described, for example in U.S. Pat. Ser. No. 3,741,657 of Andringa for "Differential Laser Gyro System". The addition of an intercavity element increases cavity losses. This is detrimental to gyro performance and a preferred method for producing reciprocal splitting is the use of a non-planar light path which produces different round-trip shase shifts for LCP and RCP light and, hence, different lasing frequencies. This method is described in Ser. Nos. 4,229,106 of Dorschner et al. for "Electromagnetic Ring Resonator" and 4,585,501 of Smith et al. for "Laser Gyroscope System".
With the reciprocal splitting technique in operation, the two gyros of the multioscillator configuration can operate independently but each will still be subject to the lock-in phenomenon. Unlike the mechanically dithered gyro in which an "a.c." bias is appied via the dither, the multioscillator circumvents this problem by applying a "d.c." bias to the two gyros so that each operates about a point far removed from dead band. This bias is known as "nonreciprocal splitting" and is accomplished by introducing a Faraday rotation into the cavity. When circularly polarized light passes through a Faraday rotator, it experiences a phase shift that depends upon the direction of propagation through the rotator. In such a manner the clockwise and counterclockwise beams in each gyro experience different phase shifts and thus lase at different frequencies. Typical values for the nonreciprocal splitting in a multioscillator are much smaller (about 1 MHz) than the reciprocal splitting. Nonreciprocal splitting can be achieved by use of an intracavity element, made of suitable glass, mounted within an axial magnetic field such as that described in the above-referenced United States patent of Andringa or by surrounding the gaseous gain medium of the cavity by an axial magnetic field as described in the Dorschner et. al. patent.
When nonreciprocal splitting is applied to the multioscillator in the prescribed manner, the resulting bias shift in the left circularly polarized gyro is equal but opposite in sign to the bias shift in the right circularly polarized gyro. Thus, when the outputs of the two gyros are summed, the resultant signal is doubly sensitive to body rotation but independent of the magnitude of the applied bias. In this way, the differential nature of the multioscillator makes it inherently insensitive to bias variations that can be caused, by example, by changes in magnetic field, temperature or the like, which have proved to be a major problem in single gyro, two-mode designs that utilize a d.c. bias.
Navigation systems must measure spacedependent variables, such as rotation, with respect to (or about) a set of three orthogonal axes. The foregoing description of a multioscillator does not address the problems inherent in attempting to achieve a practical sensor that is sensitive about three measuring or input axes. The design of a three-axis multioscillator or, in fact, any ring laser, that is sufficiently compact and realizable in a manufacturing sense is beset by numerous difficulties. In the operation of a ring laser the chosen fill gas(es) necessarily interact with applied electrical fields to produce the desired lasing action. Thus,the design of any ring laser gyroscope must provide for the positioning of anodes and cathodes in addition to locating mirror faces and internal bores.
A sensor designer must recognize the problems posed by a device whose operation relies upon the generation of current flows in a gaseous meduim. Unavoidable gas flows within the laser cavity can prove quite deleterious to the long-term operation of the device. So-called Langmuir flow effects can degrade laser performance considerably, producing, inter alia, unwanted thermal bias. Such effects have been compensated to varying extents in some single axis devices by the symmetrical placement of a plurality of electrodes about the body of the instrument. Generally this has implied the use of numerous electrodes. Thus, both the Dorschner et al. and the Smith et al. patents employ multiple anode arrangements while the planar multioscillator of Andringa utilizes two cathodes and a single anode for measuring rotation about a single axis.
The United States patents of Stiles et al., (Ser. No. 4,477,188) and Simms (Ser. No. 4,407,583) disclose the incorporation of three planar gyro cavities in to a single block. The expansion of a ring laser concept to a unit for measuring rotation about three orthogonal axes necessarily complicates the problem of providing a suitable arrangement of electrodes. The Stiles et al. device utilizes six anodes and two cathodes while the Simms apparatus includes six anodes and a single cathode. The use of a considerable number of electrodes substantially complicates instrument design. Each electrode must be sealably secured to (or within) the gyro frame in such a manner that the device remains airtight. This may add significant difficulties to the fabrication process.
The physical size of the electrodes complicates device design. A large number of electrodes will consume a correspondingly-large percentage of the frame's surface area for mounting. The size and shape of the block-frame may not be reducible to a sufficient extent to prevent arcing or other unwanted electrical interactions therebetween. Thus, the design of a ring laser rotational rate sensor that is sensitive to rotation about three orthogonal axes is significantly complicated by unavoidable effects of gas flows.
Further, the capabilities (i.e. sensitivity) and price of the instrument are functions of the size of the block-frame. A design that requires added surface area for separation of electrodes necessarily adds to the cost of the instrument. Such added cost partially defeats the compactness advantages of a three axes-in-one block device and may render the design inappropriate for singleuse applications, such as guided missiles, wherein the premium is on economy and accuracy is not critical.